1. Field of the Invention
The present invention relates to a drive control apparatus and drive control method for a display panel for displaying an image signal outputted from a computer, TV image signal or the like.
2. Description of the Related Art
Conventionally, as a method for reducing electromagnetic interference (EMI), for example, patent document 1 (Japanese Patent Application Laid-Open (JP-A) No. 8-320665), patent document 2 (JP-A No. 9-98152), patent document 3 (JP-A No. 9-232944) and patent document 4 (JP-A No. 9-289527) have been proposed.
According to the above-mentioned conventional EMI reduction method, a system clock is frequency-modulated so as to spread the spectrum of harmonics of the system clock thereby to reduce EMI spectrum amplitude which is measured in a relatively wide band.
According to the patent document 1, when digital data is transferred to a flat panel display, the phase of the data and its data transfer clock is changed over so as to reduce the EMI accompanied by the data transfer.
The patent document 2 has disclosed that a reference frequency clock is frequency-modulated and converted to an output of a spread spectrum generating apparatus. Further, a specific frequency modulation method has been explained. Additionally, this document has disclosed that deterioration level is low if each sweep to a laser beam printer or a video display for scanning synchronizes with a same point in spread spectrum.
The patent document 3 has described a method for frequency-modulating an output clock (system clock) digitally from its source clock.
The patent document 4 has disclosed a method for frequency-modulating a base signal to generate a modulated clock reference signal which is a system clock for reducing the EMI. Regarding the synchronizing signal of a display unit, this document has disclosed that the frequency of a modulated waveform is matched with the horizontal retrace period of the display unit in order to suppress a deviation of a horizontal position displayed on each scanning line.
The above-mentioned patent documents have disclosed a method for damping harmonics of the system clock of an electronic apparatus, a method for damping harmonics of data transfer, particularly, a modulation method for frequency-modulating the system clock.
However, it has been found that even if the phase of data transfer clock is changed over, some kind of unnecessary radiation critical for the EMI, which is emitted from the display panel, cannot be suppressed. This will be described in detail below.
The drive control method for the display panel includes some kinds of luminance modulation methods for determining the display luminance (gradation level) for a single pixel. The first method of them is a method for modulating the amplitude of a voltage applied to a modulation device of pixel and the second method is a method for modulating the quantity of current to be supplied to the modulation device of pixel. Additionally, other method is available for controlling based on the length of light emission period in a selecting period of that pixel and still other method is also available in which the former method is combined with the method of modulating voltage or current. These methods are called pulse width modulation method.
According to the pulse width modulation method, modulation clock (PCLK) dedicated for luminance modulation is prepared separately from data transfer clock. At least the pulse width of a modulation signal waveform is determined synchronously with this modulation clock.
It has been found that when the unnecessary radiation (EMI) of an image display unit is measured by driving a matrix panel which is a display panel according to this pulse width modulation method, the level of harmonics component of the modulation clock (PCLK) may exceed a legal standard value, for example, class B provided by voluntary control council for interference by information technology equipment (VCCI).
The modulation clock (PCLK) is determined based on the length of a time (selecting time) for selecting a row wiring, the quantity of gradations to be displayed, modulation method and the like, and as described later, it is set to near 10 MHz, at least about several MHz—40 MHz. If the screen size of matrix panel is about 40-inch 16:9, the length of the modulation wiring is about 0.5 m and if the capacity between the wires is considered, there is a possibility that resonance may occur at several hundreds MHz. Further, the harmonics component decreases gradually as the order of the harmonics increases with respect to the basic wave as estimated from Fourier transformation of rectangular wave. The unnecessary radiation emitted as a result is expanded from several tens MHz which is harmonics component of the modulation clock (PCLK) up to several hundreds MHz. Actually, various factors such as frequency of the modulation clock (PCLK) and the screen size of the matrix panel are related to the unnecessary radiation and in the matrix panel image display unit which the inventor of the present invention and others measured on, harmonic components of the modulation clock (PCLK) from 100 MHz to 400 MHz are largely detected.
Further, most of the detected harmonic components is refrained from being detected if a drive circuit which receives an input of luminance data and modulated clock so as to output a modulation signal to the matrix panel is separated from the matrix panel. An influence of the unnecessary radiation from the matrix panel originated from data transfer clock is very small as compared to that of the unnecessary radiation by the modulation clock and this is not so serious as that of the modulation clock.
That is, according to knowledge of the present inventor, it has been found that the unnecessary radiation emitted from the matrix panel cannot be eliminated sufficiently even if the data transfer clock or system clock is subjected to frequency modulation like the conventional EMI countermeasure.
The EMI emitted from the display panel can be reduced by shielding an entire case including the display panel with a conductive member. Although general electronic apparatus can be shielded with a metallic member or the like, the display portion on the front face of the matrix panel in an image display unit needs to be shielded with a member having a high electric conductivity which does not sacrifice its optical characteristic (that is, colorless transparent). However, the member having a high electric conductivity which does not sacrifice the optical characteristic is expensive.
Thus, the inventor has considered a method for reducing the harmonic component of the modulation clock (PCLK) by adding such a member as ferrite core for removing the harmonic component between the output of a modulator (drive circuit) and the modulation wiring. However, in case where the screen size of the matrix panel is 40 inch 16:9, adding ferrite cores to each of several thousands modulation wiring connected in a width of about 1 m is actually difficult from viewpoints of their mounting and boosts manufacturing cost. For this reason, this method is difficult to adopt to the image display unit of a TV unit which is a civilian product for commercial reasons.